To obtain a better understanding of the contribution of the present invention to the state of the art in question, it should be observed that the rate at which a solution of phosphoric acid reacts with phosphate depends, amongst other things, on a good distribution of phosphate grains in the acid solution. The function of this reaction is to transform the phosphorous contained in the ore in a form that cannot be assimilated by plants into another form that can be assimilated.
Inserting phosphate directly into a reaction tank, i.e. into a reactor having a stirred reaction tank, would give rise:
firstly to a drop in the rate at which phosphate is converted because a portion of the phosphate would leave the reactor without reacting; and
secondly to losses due to phosphate escaping in the waste gases.
That is why a premixer is used to mix the granulated phosphate and the phosphoric acid solution prior to entering the reaction tank. However, although it is true that a conventional premixer enables the phosphate to be wetted, use thereof gives rise in turn to problems such as blocking or clogging of the premixer and to gases and vapors being given off thereby, thus interferring with operation and requiring frequent unclogging to be performed.
An object of the present invention is to eliminate these drawbacks of the methods and installations used in the prior art and it seeks to provide a method and an installation for producing granulated TSP that are substantially improved, that satisfy practical requirements better than previously known methods and installations, in particular in that:
1) present improved granulation efficiency ensuring good physical quality to the finished product with a recycling rate close to three;
2) only a small amount of dust is produced and it is suitable for proper disposal with anti-pollution standards being guaranteed in each case;
3) only a small amount of energy is required to implement the method and the equipment required is simple, reliable, and compact;
4) the method guarantees an increase in the capacity of units already in service in the range 30% to 100%;
5) above all, the method makes it possible, by using appropriate equipments, to avoid the problems usually encountered in the slurry circuits, in particular blocking or clogging by the slurry setting, and also the presence of pockets of gas and pumping;
6) the solution adopted enables starting and stopping to be performed rapidly while also making production easy to run;
7) the method can be adapted to installations that are already in service; and
8) the solution adopted is also remarkable in that it prevents return of the reaction gases.